XSS (Cross-Site Scripting) is a type of web application vulnerability that allows attackers to inject malicious scripts into a webpage’s code. These scripts are executed in the user’s browser and can be used to steal data, hijack sessions, alter page content, or perform actions on behalf of the user.
What is XSS
The abbreviation XSS stands for Cross-Site Scripting — the letter “X” is used instead of “C” to avoid confusion with CSS. The XSS vulnerability occurs when a web application improperly handles user input — such as comments, form data, or URL parameters — and embeds it into the HTML page without proper sanitization.
As a result, an attacker can inject JavaScript code into a page that executes in other users’ browsers. This allows them to steal cookies, access personal data, or even gain control over user accounts.
How XSS Works
An XSS attack follows a simple principle: the attacker sends malicious code to the web server, which then returns it to the user as part of the webpage. When the browser loads the page, it executes the injected script as if it were legitimate content.
There are three main types of XSS:
- Reflected XSS – the malicious code is passed through a URL or form and executed when the page loads.
- Stored XSS – the code is saved on the server (for example, in a database or comments) and executed each time the infected page is viewed.
- DOM-based XSS – the script is injected and executed on the client side via the Document Object Model, without interacting with the server.
When successful, an XSS attack can steal session cookies, inject phishing interfaces, or redirect users to fake websites.
Use and Consequences
XSS is not used for legitimate purposes — it is strictly a vulnerability that developers and system administrators must prevent. It often appears in feedback forms, search fields, user dashboards, and comment sections where user input is not properly sanitized.
The consequences of XSS depend on the level of access and the nature of the website:
- On user portals – theft of personal data, logins, and passwords.
- On corporate systems – session hijacking and access to internal resources.
- On financial platforms – tampering with payment forms or injecting malicious links.
How to Prevent XSS
To protect against XSS, it’s essential to sanitize and escape all user-provided data.
Key measures include:
- Validating and cleaning input data on the server side.
- Using Content Security Policy (CSP) to restrict script execution.
- Avoiding unsafe functions such as innerHTML.
- Employing frameworks that automatically escape user input (e.g., React, Angular, Django).
Regular security testing and code audits help identify vulnerabilities in time and reduce the risk of system compromise.
Example
A user posts a comment on a website containing the code <script>document.cookie</script>. If the site does not filter input, this script will be stored and executed every time the comment is viewed. As a result, the attacker can obtain other users’ cookies and use them to log in under their accounts.